The present invention relates to an ultrafine particle organic pigment color ink, particularly, to an ink suitable for ink jet recording carried out by squirting ink droplets from a printing head, and, more particularly, to a pigment color ink which can give printed images of high quality and excellent light resistance.
In the conventional ink jet recording, dye inks prepared by dissolving various dyes in water or mixed solvents of water and organic solvents have been used.
However, dye inks have the defects that they are apt to blot on the recording paper to deteriorate the quality of the resulting printed images and the resulting images are inferior in light resistance and water resistance.
Under the circumstances, investigation has been made on pigment inks which blot less and can provide printed images excellent in quality and high in light resistance and water resistance. However, pigment inks suffer from problems in dispersibility and dispersion stability of pigment particles in the ink and, furthermore, the problem of clogging the printing head.
JP-A-56-147871 proposes a method for the solution of these problems. That is, it discloses an aqueous medium comprising at least a pigment, a polymer dispersant and a nonionic surface active agent.
However, with recent progress of ink jet printers for producing further minute images, the pigments having a particle diameter of several hundred .mu.m to several .mu.m as described in JP-A-56-147871 often cause clogging of printing head of the ink jet printer and result in insufficient image quality.
At present, black inks comprising carbon black are practically used, but inks having chromatic colors such as cyan, magenta and yellow have not yet been put to practical use. It is considered that this is because carbon black of small primary particle diameter can be easily obtained and, further, it can be relatively easily dispersed due to the presence of functional groups on the particle surface while organic pigments of chromatic colors are large in primary particle diameter and, in addition, they can hardly be made into fine particles and can hardly be highly stabilized in dispersibility owing to the absence of functional groups on the particle surface.
That is, the dispersion method employed for carbon black is not necessarily effective for organic pigments of chromatic colors. For these reasons, color inks comprising organic pigments having chromatic colors have been delayed in practical use and have not yet become popular.
On the other hand, even if organic pigments of chromatic colors can be made into fine particles, in the case of the particles being merely made fine, viscosity of the pigment inks increases and, furthermore, dispersion stability of once settled pigment is low and it forms so-called hard cake which is difficult to redisperse. For a pigment ink used for ink jet recording, it is essential for prevention of pigment from settling to make it into fine particles, but this causes problems such as increase in viscosity of ink and lacking of dispersion stability.